1. Field of the Invention
The present invention relates to a bidirectional switching device for switching alternating current and an AC semiconductor active fuse employing the bidirectional switching device.
2. Description of the Related Art
FIG. 1 shows a direct current supply/control apparatus according to a related art. The direct current supply/control apparatus has a switching element QF having a temperature sensor. The switching element QF controls the supply of power from a DC power source to a load. The DC power source 101 supplies a DC output voltage VB. The power source 101 is connected to an end of a shunt resistor RS. The other end of the shunt resistor RS is connected to a drain electrode D of the switching element QF whose source electrode S is connected to the load 102. The load 102 is, for example, a headlight or a power window motor of a vehicle. The bidirectional switching device also has a driver 901 for detecting a current flowing through the shunt resistor RS and controlling the switching element QF accordingly, an A/D converter 902, and a microcomputer (CPU) 903 for turning on and off a drive signal for the switching element QF according to the current detected by the driver 901 When the temperature of the switching element QF increases, the switching element QF is turned off.
A Zener diode ZD1 is connected between the gate and source of a power element QM serving as a main semiconductor element of the switching element QF. The Zener diode ZD1 keeps a voltage of 12 V between the gate electrode G and source electrode S of the switching element QF to bypass an overvoltage so that the overvoltage may not be applied to the true gate TG of the switching element QF. The driver 901 has differential amplifiers 911 and 913 serving as a current monitor circuit, a differential amplifier 912 serving as a current limiter, and a charge pump 915. The driver 901 incorporates a driver 914 for receiving an ON/OFF control signal from the microcomputer 903 and an overcurrent signal from the current limiter, and according to these signals, driving the true gate TG of the switching element QF through an internal resistor RG. If an overcurrent exceeding an upper limit is detected by the differential amplifier 912 according to a voltage drop across the shunt resistor RS, the driver 914 turns off the switching element QF. If the overcurrent drops below a lower limit, the driver 914 turns on the switching element QF. On the other hand, the microcomputer 903 always monitors a current through the current monitor circuit made of the differential amplifiers 911 and 913. Upon detecting an abnormal current exceeding a normal level, the microcomputer 903 issues an OFF signal to the switching element QF to turn off the switching element QF. If the temperature of the switching element QF exceeds a predetermined level before the microcomputer 903 issues the OFF signal, a temperature sensor 121 issues a signal to turn off the switching element QF.
To detect a current, the related art must have the shunt resistor RS in a power supply cable. If a large current flows through the shunt resistor RS, the shunt resistor RS will cause a large heat dissipation The large heat dissipation is waste of electric energy and needs a cooler, which complicates and enlarges the supply/control apparatus.
The direct current supply/control apparatus of the related art may work on a dead short that occurs in the load 102 or wiring to produce a large short-circuit current. However, the supply/control apparatus unsatisfactorily works on an incomplete short circuit failure having a certain extent of short-circuit resistance to produce a weak short-circuit current. Only way for the related art to cope with such incomplete short circuit failures is to detect an abnormal current caused by the short circuit failure with the use of the microcomputer 903 and current monitor circuit and turn off the switching element QF by the microcomputer 903. The microcomputer 903 is expensive and is slow to respond to such an abnormal current.
The shunt resistor RS, A/D converter 902, and microcomputer 903 that are imperative for the related art need a large space and are expensive, to increase the size and cost of the supply/control apparatus.
In addition to these problems, there is no related art that provides a bidirectional switching device or xe2x80x9can AC semiconductor active fusexe2x80x9d capable of operating on an AC power supply cable to disconnect the AC power supply cable upon detecting an abnormal current.
The reason why there is no bidirectional switching device or AC semiconductor active fuse is mainly because a control circuit for controlling a bidirectional switching device inserted in an AC power supply cable is difficult to design A control circuit for handling small signals usually operates on a voltage of, for example, 6 V, and it is very difficult to design a control circuit that withstands a commercial AC voltage of about 100-130 V Further difficulty lies in monolithically integrating a bidirectional switching device and its control circuit into a power device.
An object of the present invention is to provide a bidirectional switching device capable of serving for an AC power supply cable, detecting an abnormal current, and disconnecting the AC power supply cable accordingly.
Another object of the present invention is to provide a bidirectional switching device capable of detecting alternating current without a shunt resistor in an AC power supply cable.
Still another object of the present invention is to provide a bidirectional switching device that is easy to integrate and is manufacturable at low cost.
Still another object of the present invention is to provide an AC semiconductor active fuse capable of serving for an AC power supply cable.
Still another object of the present invention is to provide an AC semiconductor active fuse capable of suppressing heat dissipation in an AC power supply cable and efficiently supplying AC power,
Still another object of the present invention is to provide an AC semiconductor active fuse that is small and light and needs no labor for replacement.
Still another object of the present invention is to provide an AC semiconductor active fuse capable of speedily responding to an abnormal current caused by an incomplete short circuit failure having a certain extent of short-circuit resistance.
Still another object of the present invention is to provide an AC semiconductor active fuse whose breaking speed for an incomplete short circuit failure is adjustable.
Still another object of the present invention is to provide a structure for a semiconductor switch employed by an AC semiconductor active fuse, to reduce the size and cost of the fuse.
Still another object of the present invention is to provide an AC semiconductor active fuse having a control circuit that withstands the commercial AC voltage.
Still another object of the present invention is to provide a bidirectional switching device and a control circuit that controls the bidirectional switching device and withstands the commercial AC voltage, to form monolithically an AC semiconductor active fuse on a semiconductor chip.
Still another object of the present invention is to provide an AC semiconductor active fuse capable of detecting an abnormal current without intricate, expensive hardware such as a microcomputer and being small, light, and inexpensive.
Still another object of the present invention is to provide an AC semiconductor active fuse having uniform characteristics, employing no precision capacitors or resistors, and minimizing detection errors.
Still another object of the present invention is to provide an AC semiconductor active fuse that needs no external capacitor and is small and manufacturable at low cost.
Still another object of the present invention is to provide an AC semiconductor active fuse that is compact to improve space efficiency in a semiconductor chip and is manufacturable at low cost.
In order to accomplish the objects, a first feature of the present invention inheres in a bidirectional switching device for an AC semiconductor active fuse, having a novel structure. The bidirectional switching device consists of a p-channel first main semiconductor element and an n-channel second main semiconductor element. The first main semiconductor element has a first main electrode connected to an ungrounded side of an AC power source, a second main electrode opposing to the first main electrode, and a first control electrode for controlling a main current flowing between the first and second main electrodes. The first main semiconductor element contains a first parasitic diode whose cathode region is connected to the first main electrode and whose anode region is connected to the second main electrode. The second main semiconductor element has a third main electrode connected to the second main electrode, a fourth main electrode opposing to the third main electrode and connected to a load, and a second control electrode for controlling a main current flowing between the third and fourth main electrodes. The second main semiconductor element contains a second parasitic diode whose anode region is connected to the third main electrode and whose cathode region is connected to the fourth main electrode. The first and second main semiconductor elements may be vertical-type power MOS transistors having a DMOS, VMOS, or UMOS structure. Alternatively, the first and second main semiconductor elements may be MOS static induction transistors (SITs) having a similar structure. These transistors are preferable because they increase the areas of the first and second parasitic diodes. The first and second main semiconductor elements may be MOS composite device s such as emitter switched thyristors (EST) and MOS controlled thyristors (MCT). Instead, the first and second main semiconductor elements may be insulated gate power device s such as insulated gate bipolar transistors (IQBTs) Further, the first and second main semiconductor elements may be another insulated gate transistors such as metal-insulator-semiconductor (MIS) transistors, which may include high electron mobility transistors (HEMTs). If the first and second main semiconductor elements are always used with reversely-biased gates, they may be junction FETs, junctions SITs, or SI thyristors. Double-gate SI thyristors realize bidirectional switching with a low ON voltage, The first and second parasitic diodes correspond to parasitic p-n junction diodes structurally contained in the above-mentioned semiconductor elements or device s.
According to the first feature, the first and second control electrodes are grounded through resistors when energized. When the ungrounded side of the AC power source increases to be positive, the potential of the control electrode of the first main semiconductor element decreases with respect to the potential of the first main electrode, and the potential of the control electrode of the second main semiconductor element decreases with respect to the potential of the third main electrode. As a result, the p-channel first main semiconductor element turns on, and the n-channel second main semiconductor element keeps off. The first main electrode is an emitter electrode of the IGBT, a source electrode of a MOS transistor, a cathode electrode of the EST, MCT, or SI thyristor, or an equivalent main electrode of a semiconductor element equivalent to any one of these semiconductor elements. The second main electrode is a collector electrode of the IGBT, a drain electrode of the MOS transistor, or an anode electrode of the EST, MCT, or SI thyristor. Similarly, the third main electrode is an emitter electrode of the IGBT, a source electrode of the MOS transistor, or a cathode electrode of the EST, MCT, or SI thyristor. The fourth main electrode is a collector electrode of the IGBT, a drain electrode of the MOS transistor, or an anode electrode of the EST, MCT, or SI thyristor. According to the first feature, a current from the ungrounded side of the AC power source passes through the first and second main semiconductor elements to the load and to the ground, even if the second main semiconductor element is in the nonconducting state, because there is the second parasitic diode. When the ungrounded side of the AC power source decreases to be negative, the second main semiconductor element turns on to reversely pass a current through the second main semiconductor element and first parasitic diode.
Due to the first and second parasitic diodes, the first and second main semiconductor elements of the first feature function as reverse-conducting semiconductor elements. The reverse-conducting semiconductor elements may use forward and reverse current paths when employed for a bidirectional switching device. The first and second parasitic diodes are structurally formed in large areas in vertical-type semiconductor elements having a DMOS, VMOS, or UMOS structure, to reduce ON resistance. The vertical-type semiconductor elements may have a structure in which a buried electrode region is guided to the surface with a sinker region, which is a highly conductive semiconductor region. In this case, the first and second main semiconductor elements may be connected in series without increasing a conduction loss. Using the first and second parasitic diodes may reduce the number of parts of an overcurrent controller of an AC semiconductor active fuse and the whole size of the fuse.
The bidirectional switching device of the first feature may further have a first reference semiconductor element having a fifth main electrode connected to the first main electrode, a third control electrode connected to the first control electrode, and a sixth main electrode, and a second reference semiconductor element having a seventh main electrode connected to the third main electrode, a fourth control electrode connected to the second control electrode, and an eighth main electrode.
According to the first feature, the first main semiconductor element, first reference semiconductor element, second main semiconductor element, and second reference semiconductor element may monolithically be merged on a single semiconductor substrate, to reduce the size and space of the switching device. This enables the bidirectional switching device to be mass-produced to reduce the cost thereof. The first main semiconductor element, first reference semiconductor element, second main semiconductor element, and second reference semiconductor element may separately be formed in island-like semiconductor areas that are isolated and discrete In this case, the second, fourth, sixth, and eighth main electrodes are formed as buried regions at the bottoms of the island-like semiconductor areas.
The first main semiconductor element, first reference semiconductor element, second main semiconductor element, and second reference semiconductor element may be formed in individual modules, which are arranged in a single package. In this case, the first main semiconductor element, first reference semiconductor element, second main semiconductor element, and second reference semiconductor element may be formed on separate conductive plates arranged on the surface of a single package base. The second, fourth, sixth, and eighth main electrodes are directly connected to the respective conductive plates, so that they may separately be led to the outside. It is convenient to connect the second and third main electrodes to each other as an internal structure in a package.
A second feature of the present invention lies in a bidirectional switching device for an AC semiconductor active fuse The bidirectional switching device has an n-channel first main semiconductor element and an n-channel second main semiconductor element The first main semiconductor element has a first main electrode connected to an ungrounded side of an AC power source, a second main electrode opposing to the first main electrode, and a first control electrode for controlling a main current flowing between the first and second main electrodes. The first control electrode is connected to a first driver that is stepped up by a charge pump. The first main semiconductor element contains a first parasitic diode whose cathode region is connected to the first main electrode and whose anode region is connected to the second main electrode. The second main semiconductor element has a third main electrode connected to the second main electrode, a fourth main electrode opposing to the third main electrode and connected to a load, and a second control electrode for controlling a main current flowing between the third and fourth main electrodes. The second control electrode is connected to a second driver that is different from the first driver. The second main semiconductor element contains a second parasitic diode whose anode region is connected to the third main electrode and whose cathode region is connected to the fourth main electrode.
According to the second feature, the first main electrode is a collector electrode of an IGBT, a drain electrode of a MOS transistor, an anode electrode of an EST, MCT, or SI thyristor, or an equivalent main electrode of a semiconductor element equivalent to any one of these semiconductor elements. The second main electrode is an emitter electrode of the IGBT, a source electrode of the MOS transistor, or a cathode electrode of the EST, MCT, or SI thyristor. These polarities are opposite to those of the first main semiconductor element of the first feature. On the other hand, the polarities of the second main semiconductor element are the same as those of the second main semiconductor element of the first feature. The third main electrode is the emitter electrode of the IGBT, the source electrode of the MOS transistor, or the cathode electrode of then EST, MCT, or SI thyristor. The fourth main electrode is the collector electrode of the IGBT, the drain electrode of the MOS transistor or the anode electrode of the EST, MCT, or SI thyristor. When energized, the first control electrode is grounded through a resistor. When the ungrounded side of the AC power source increases to be positive, the potential of the control electrode of the first main semiconductor element decreases with respect to the potential of the first main electrode, and therefore, the n-channel first main semiconductor element is unable to turn on. Accordingly, the second feature connects the first control electrode to the first driver that is stepped up by the charge pump, thereby increasing the potential of the first control electrode with respect to the potential of the second main electrode. This turns on the first main semiconductor element When energized, the second control electrode is grounded through a resistor, and the potential of the control electrode of the second main semiconductor element decreases with respect to the potential of the third main electrode. As a result, the n-channel second main semiconductor element is in the nonconducting state. Even so, the second main semiconductor element contains the second parasitic diode, which passes a current from the ungrounded side of the AC power source through the first and second main semiconductor elements and the load and to the ground. When the ungrounded side of the AC power source decreases to be negative, a current reversely flows through the second main semiconductor element that is ON and the first parasitic diode.
A third feature of the present invention provides an AC semiconductor active fuse having a p-channel first main semiconductor element, an n-channel second main semiconductor element, a first reference semiconductor element, and a second reference semiconductor element. The first main semiconductor element has a first main electrode connected to an ungrounded side of the AC power source, a second main electrode opposing to the first main electrode, and a first control electrode for controlling a main current flowing between the first and second main electrodes. The first main semiconductor element contains a first parasitic diode whose cathode region is connected to the first main electrode and whose anode region is connected to the second main electrode. The second main semiconductor element has a third main electrode connected to the second main electrode, a fourth main electrode opposing to the third main electrode and connected to a load, and a second control electrode for controlling a main current flowing between the third and fourth main electrodes. The second main semiconductor element contains a second parasitic diode whose anode region is connected to the third main electrode and whose cathode region is connected to the fourth main electrode. The first reference semiconductor element has a fifth main electrode connected to the first main electrode, a third control electrode connected to the first control electrode, and a sixth main electrode. The second reference semiconductor element has a seventh main electrode connected to the third main electrode, a fourth control electrode connected to the second control electrode, and an eighth main electrode. The AC semiconductor active fuse further has a first comparator for comparing voltages of the second and sixth main electrodes with each other, and a second comparator for comparing voltages of the fourth and eighth main electrodes with each other.
The first and second main semiconductor elements are each a semiconductor element for controlling a main current flowing through a power supply cable. The first reference semiconductor element, first comparator, etc., form a first control circuit for detecting an abnormal current flowing through load of the first main semiconductor element and turning on and off the first main semiconductor element in response to the detected abnormal current, to cause current oscillations that turn off the first main semiconductor element. More precisely, the first control circuit is a first control voltage supply circuit for providing a control voltage to the first and third control electrodes in response to the output of the first comparator. The first main semiconductor element and the first control circuit can form a power IC. The second reference semiconductor element, second comparator, etc., form a second control circuit for detecting an abnormal current flowing through load of the second main semiconductor element and turning on and off the second main semiconductor element accordingly. Upon detecting an abnormal current, the second control circuit turns on and off the second main semiconductor element to cause current oscillations, which turn off the second main semiconductor element. The second control circuit is a second control voltage supply circuit that supplies a control voltage to the second and fourth control electrodes in response to the output of the second comparator. The second main semiconductor element and the second control circuit can form a power IC. That is, the first and second main semiconductor elements and the first and second control circuits could be merged on a single semiconductor chip to form a power IC.
To produce current oscillations to make the first and second main semiconductor elements nonconducting state, or current blocking state so as to break a main current flowing through a power supply cable, a temperature sensor is disposed around the first and second main semiconductor elements. The temperature sensor detects a temperature increase promoted by current oscillations and turns off the first and second main semiconductor elements. Alternatively, the number of current oscillations may be counted, and when the counted number reaches a predetermined value, the first and second main semiconductor elements are turned off to block a main current flowing through a power supply cable. A simplest way to count the number of current oscillations is to measure charge accumulated at a capacitor, i.e, a terminal voltage of the capacitor.
The AC semiconductor active fuse of the third feature is capable of detecting an overcurrent without a shunt resistor, which was connected in series with a conventional power supply cable. Accordingly, the third feature reduces heat dissipation and a conduction loss. The third feature is capable of simply and speedily detecting not only an overcurrent caused by a dead short but also an abnormal current caused by an incomplete short circuit failure having a certain extent of short-circuit resistance, and cutting a main current flowing through a power supply cable. In addition, the third feature is capable of detecting and controlling an overcurrent in a power supply cable without a microcomputer, thereby greatly reducing the size and cost of the power supply system. The third feature employs no classical metallic fuse, which melts when current exceeds specific amperage so as to open the circuit, reducing the size and weight of a power supply system. The third feature eliminates labor for replacing blown fuses from the power supply system.
According to the third feature, a terminal voltage between the first and second main electrodes of the first main semiconductor element has an OFF-to-ON voltage characteristic curve (a fall characteristic curve) that is dependent on the conditions of a power supply cable and load. Similarly, the fall characteristic curve of a terminal voltage between the third and fourth electrodes of the second main semiconductor element is dependent on the conditions of the power supply cable and load. Depending on the wiring inductance of the power supply cable and a time constant determined by wiring resistance or short-circuit resistance, the fall characteristic curves change. For example, the fall characteristic curves quickly converge below a predetermined voltage under a normal state having no short circuit failure in the power supply cable. If a dead short occurs in the power supply cable, the fall characteristic curves never converge under the predetermined voltage. If there is an incomplete short circuit failure having a certain extent of short-circuit resistance in the power supply cable, the fall characteristic curves take a long time to converge below the predetermined voltage. The AC semiconductor active fuse of the third feature uses such voltage characteristics of semiconductor elements in an OFF-to-ON transient period. Namely, the third feature detects the difference between a terminal voltage of the first main semiconductor element and a reference terminal voltage of the first reference semiconductor element, or the difference between a terminal voltage of the second main semiconductor element and a reference terminal voltage of the second reference semiconductor element, and determines a deviation of the terminal voltage (i.e., a current in the power supply cable) of the first or second main semiconductor element that is inserted in the power supply cable, from a voltage corresponding to a normal state. By connecting a plurality of first and second main semiconductor elements in parallel according to their rated currents, it is possible to handle a large current. To detect a weak current, a voltage corresponding to the weak current is set in the AC Semiconductor active fuse of the third feature. The third feature is capable of optionally setting a breaking speed with respect to an incomplete short circuit failure. Unlike the already mentioned related art that detects an overcurrent by comparison with a threshold at set timing, the third feature detects an overcurrent according to a change in the transient characteristics of a terminal voltage of the first or second main semiconductor element, so that the third feature may eliminate some parts such as capacitors and resistors, thereby minimizing detection errors caused by parts variations. In addition, the third feature eliminates an external capacitor from a semiconductor chip on which the AC semiconductor active fuse is packaged, thereby greatly reducing the size and cost of the semiconductor active fuse.
The AC semiconductor active fuse of the third feature detects a current without a shunt resistor, which was connected in series with a conventional power supply cable. Accordingly, the third feature minimizes heat dissipation and effectively uses electric energy. Unlike the classical passive fuse, the semiconductor active fuse of the third feature is capable of handling not only an overcurrent caused by a dead short but also an abnormal current caused by an incomplete short circuit failure having a certain extent of short-circuit resistance. Further, the third feature needs no microcomputer for controlling ON/OFF operations. The third feature employs a simple hardware circuit for controlling ON/OFF operations. The AC semiconductor active fuse of the third feature needs a small packaging space and greatly reduces the cost of at AC power system.
According to the third feature, the first main semiconductor element may be composed of N1 first unit cells and the first reference semiconductor element of N2 first unit cells with N1 greater than  greater than N2. Also, the second main semiconductor element may be composed of N3 second unit cells and the second reference semiconductor element of N4 second unit cells with N3 greater than  greater than N4. Namely, each of the first and second main semiconductor elements are a power element composed of unit cells connected in parallel with one another, to realize a multi-channel structure to provide a rated current handling capability. The current handling capability of each of the first and second reference semiconductor elements is set to be smaller than that of the corresponding main semiconductor element by adjusting the number of parallel-connected unit cells that form the main and reference semiconductor elements The numbers N1 and N2 of unit cells determine a current dividing ratio of N1:N2. And the numbers N3 and N4 of unit cells determine a current dividing ratio of N3:N4 For example, N2=1, and N1=1000. In this case, the ratio of the channel width of the first reference semiconductor element to that of the first main semiconductor element is 1:1000, and a current dividing ratio is determined accordingly. By making the circuit configurations in this way, the sizes of reference semiconductor elements are minimized, to reduce the size and cost of a semiconductor chip on which the first and second semiconductor active fuses of the third feature are merged.
According to the third feature, the first main semiconductor element, first reference semiconductor element, second main semiconductor element, second reference semiconductor element first comparator, second comparator, and other related elements of the AC semiconductor active fuse may monolithically be integrated on a single semiconductor substrate, to reduce a packaging space. This enables the semiconductor active fuse to be mass-produced, to reduce the cost thereof. Alternatively, the first main and reference semiconductor elements and second main and reference semiconductor elements may be integrated into xe2x80x9ca power chipxe2x80x9d, and the first and second comparators and other related elements may be integrated into xe2x80x9ca control chipxe2x80x9d, to form a multi-chip module (MCM) or a hybrid IC of compact size.
Other and further objects and features of the present invention will become obvious upon an understanding of the illustrative embodiments about to be described in connection with the accompanying drawings or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employing of the invention in practice.